Apparatus for inserting sheets of material into envelopes are well known in the art: see, for example, U.S. Pat. No. 2,325,455 which discloses a "Phillipsburg" type envelope handling machine incorporating a plurality of spaced apart reciprocable gripper arm assemblies including jaw members which are each selectively operable to grip and retrieve a sheet of material from a pick-up station for later insertion into an envelope.
In most situations, it is desirable for the jaws on each of the gripper arms to grip and retrieve one sheet of material. However, in some instances, the jaws mistakenly fail to grip even the one required sheet of material and, in other instances, the jaws mistakenly grip more than one sheet of material. These conditions are typically referred to as "miss" and "double" fault conditions respectively.
A number of different detection structures have been used to detect these fault conditions. One such detection structure is shown in U.S. Pat. No. 3,885,780 which discloses a gripper arm including a lever which is attached to a movable jaw and is pivotable in one direction into contact with one switch or electrode to detect a "miss" fault condition and in the opposite direction into contact with an opposed switch or electrode to detect a "double" fault condition.
A disadvantage associated with this sensor structure is that the switch contact points on each of the arms must be manually adjusted at the beginning of every job depending upon the thickness of the insert sheets of material. The switch contact points, being high tolerance electrical components, also have a tendency over time to wear and become oxidized and highly resistive thus contributing to the inaccurate detection of fault conditions.
In an effort to overcome the problems associated with detector structures incorporating such high tolerance electrical components, gripper arms have also incorporated sensor structures such as those disclosed in, for example, U.S. Pat. Nos. 4,634,107; 5,647,583; and 5,704,246.
U.S. Pat. No. 4,634,107 discloses a gripper arm incorporating a Hall-effect sensor located adjacent the distal end of the gripper arm which cooperates with a disc-shaped magnet which is mounted to the side of the movable jaw and is positioned directly opposite the sensor. In operation, the displacement of the movable jaw in response to a "miss" or "double" fault situation causes the magnet to slide across the sensor. The "miss" or "double" fault condition is detected by measuring the change in the sensor's voltage output resulting from a change in the intensity of the magnetic flux lines in response to a change in the location of the magnet in relation to the sensor.
U.S. Pat. No. 5,647,583 discloses a gripper arm incorporating a Hall-effect sensor which is placed in the lower stationary jaw of the arm. A disc-shaped magnet is positioned immediately below the sensor. As the movable jaw is moved closer or further from the sensor in response to a "miss" or "double" fault condition, the magnetic flux lines created by the magnet are correspondingly intensified or detensified and the sensor produces a voltage output which is proportional in magnitude to the distance between the movable jaw and the sensor. The voltage outputs are fed to a computer which determines the presence of either a "miss" or "double" fault condition.
U.S. Pat. No. 5,704,246 discloses a gripper arm incorporating a lever which is operatively associated for pivotal movement in response to the displacement of the movable jaw. The lever includes an arcuate raster gauge at the distal end thereof including spaced apart raster detection marks. A pair of sensors which are secured to the side of the gripper arm opposite the raster gauge are adapted to detect a change in the location of the raster gauge marks in response to the pivotal movement of the lever in response to either a "miss" or "double" fault condition.
A disadvantage associated with the Hall-effect sensor structures disclosed in U.S. Pat. Nos. 4,697,246 and 5,647,583 is the placement and location of the magnet generally adjacent to the associated movable jaw. In applications where relatively thin sheets of material are gripped by the jaws, a "miss" or "double" fault condition results in the negligible or minute displacement of the movable jaw which, because of the generally adjacent relationship between the jaw and the magnet, results in a corresponding negligible or minute change in the intensity of the magnetic flux lines which is not always detectable by the sensor.
Moreover, in U.S. Pat. No. 5,647,583, the presence of the one or more sheets of material between the magnet and the movable jaw creates a barrier between the sensor and the jaw which reduces the magnet's ability to accurately detect a change in the position of the movable jaw.
A disadvantage of the sensor structure disclosed in U.S. Pat. No. 5,704,246 is that it requires the use of a specially manufactured arcuate raster gauge.
What is needed is a gripper arm incorporating an improved sensor and cooperating magnet assembly which provides for a high degree of detection in all applications including those applications where relatively thin sheets of materials are gripped between the jaws of the arm.